The invention relates to a process for monitoring a continuous acetic acid and/or methyl acetate production.
More precisely, the invention relates to a method of improving the monitoring and control of a process for the preparation of acetic acid and/or methyl acetate.
Of the acetic acid manufacturing processes in common use, one of the most widely used in industry is the carbonylation of methanol or, more generally, a carbonylatable derivative of methanol with carbon monoxide. This reaction is carried out in the liquid phase under carbon monoxide pressure, carbon monoxide being one of the reactants, in the presence of a homogeneous catalyst system.
The rhodium-based carbonylation process is a known process which is exploited in industry and has formed the subject of numerous articles and patents, e.g. U.S. Pat. Nos. 3,769,329 and 3,813,428.
European patents EP 618 183 and EP 618 184, and European patents EP 785 919 and EP 759 022, describe a process for the carbonylation of methanol in the presence of an iridium-based catalyst system which may also contain rhodium.
A carbonylation process based on iridium and ruthenium, currently exploited in industry, is described in European patent EP 643 034.
The aim of improving these methanol carbonylation processes was to increase the productivity of the catalysts and reduce the acetic acid manufacturing costs.
The so-called xe2x80x9clow water contentxe2x80x9d processes actually permit a considerable increase in acetic acid production, thereby limiting the level of investment required and lowering the operating costs by reducing the energy required to separate the acetic acid from the various constituents of the reaction mixture, particularly water.
Conventionally these different processes for the carbonylation of methanol in the liquid phase and in the presence of a homogeneous catalyst system are carried out in installations comprising three separate zones, as described in the article by M. J. HOWARD et al., Catalysis Today, 18 (1993) 325-354.
The reaction zone consists of a stirred continuous reactor operating under pressure (5-200 bar) and at elevated temperature (150-250xc2x0 C.).
The methanol feed and a number of recycle streams are introduced at the bottom of the reactor. The carbon monoxide is dispersed in the reactor.
The liquid reaction medium produced is then sent to the second zone, called the vaporization zone or flash zone, in which the liquid is partially vaporized at a pressure below the reaction pressure.
This creates a flash, or adiabatic expansion, in which the majority of the light constituents (methyl iodide, methyl acetate and water) are vaporized together with the acid produced.
The flash zone makes it possible to separate the gas from the liquid; the vaporized stream then passes into the third zone, called the separation zone, while the liquid stream (essentially acetic acid containing the catalyst) is recycled into the first zone.
The purification zone can comprise one or more distillation columns; it makes it possible to separate the acetic acid and/or methyl acetate from the other constituents and to recycle the streams into the reaction zone.
In addition, a gas bleed at the top of the reactor makes it possible to monitor the level of carbon monoxide partial pressure and to remove the gaseous reaction by-products together with the inert gases present in the carbon monoxide feed.
The result of optimizing the current processes for the preparation of acetic acid is to maximize the acetic acid production in already existing equipment. To do this, the reaction is carried out with the concentrations of the various constituents of the reaction medium being kept at predetermined values so as to take advantage of the most appropriate kinetic conditions.
It then becomes essential to monitor the reactor and the recycle streams.
A variety of recent articles and patents have started to tackle this area.
The paper delivered by D. Z. TOBIAS at the IEEE Conference on Advanced Process Control, VANCOUVER, Apr. 29-30 1999, entitled xe2x80x9cAdaptive Process Control of an Acetic Acid Reactorxe2x80x9d, shows the use of an external exchanger for making the material balance independent of the calorific balance, since without this exchanger it is impossible to vary the reactor output rate in order to maintain the reactor temperature, i.e. the reactor output rate is in effect dictated by the material balance.
The desired objective is to stabilize the reactor temperature in order to increase the production of the unit; the installation of a new monitoring system thus made it possible to reduce the standard deviation of the temperature from 3.6 to 0.8xc2x0 C.
In continuous operations, it is customary to supply the carbon monoxide on demand under the control of the total pressure in the reactor.
The object of European patent application EP 0 983 752 is to install a monitoring system which aims to keep the carbon monoxide flow rate below a calculated maximum value representing an acceptable maximum flow rate.
The aim of European patent application EP 0 999 198 is to maintain the composition of the reaction medium, particularly the water and methyl acetate concentrations, in the carbonylation reactor by recycling a rich acetic acid stream coming from the acid purification zone.
Patents EP 0 846 674 and FR 2 750 984 describe the optimization of the CO consumption by introducing a second carbonylation reactor (finishing effect) between the 1st reactor (reaction zone) and the flash zone (vaporization zone).
U.S. Pat. Nos. 5,352,415 and 5,374,774 describe acetic acid manufacturing processes in which the levels of the reactor and the flash zone and the water concentration in the reaction medium are monitored.
U.S. Pat. No. 5,831,120 provides a technical solution for avoiding the accumulation of water in the reaction medium to give a target water concentration in the carbonylation reactor.
These various documents teach that, in the conventional process developed by MONSANTO, the heat of reaction was extracted via expansion of the reactor output, or flash, between the reactor and the flash zone. Thus, to monitor the reactor temperature, there was a fixed relationship between the flow rate of methanol entering the reactor and the flash rate. This led to a small variability in the level of liquid in the reactor and the intermediate process streams.
With the introduction of low water content processes, this system becomes inappropriate because the flash vaporization rate increases considerably due to the fact that the quantities of water evaporated are smaller and that they have to be replaced with much larger quantities of organic products whose latent heat of vaporization is lower than that of water.
This led to the installation of cooling exchangers for absorbing part of the heat of reaction, but this in turn increased the variability of the liquid levels in the reactor and the flash zone, as well as the liquid flow rates.
This great variation in liquid flow rates, in particular, leads to deficiencies in the process because it may be necessary to decrease production in order to reduce the stream entering the purification zone.
Furthermore, these fluctuations cause variations in the water concentration in the reactor and, as the reaction kinetics depend on the water concentration in the case of low water content processes, this again increases the risks of instability of the system.
The aim of patent application EP 1 002 785 is to maintain the methyl acetate concentration in the reactor at a predetermined value by adjusting the ratio of methanol to carbon monoxide, which regulates the feed rate of methanol in the reactor.
International patent application WO 00/37405 provides a method of monitoring the process by measuring the concentrations of the various components of the reaction solution by means of an infrared analyzer and, in response thereto, adjusting the concentrations of at least the catalyst species, the methyl iodide, the methyl acetate and the water in order to optimize the acetic acid production.
Whatever the case may be, the problem of monitoring the acetic acid reactor and maintaining particularly the water and methyl acetate concentrations has been clearly defined, but a means of achieving these objectives automatically has never been suggested.
Furthermore, no document has concerned itself with the problems presented by the larger or smaller fluctuations in the carbon monoxide stream entering the reactor, or sought to overcome the disadvantages which may result there from in terms of the acetic acid production.
The object of the present invention is precisely to overcome these disadvantages by providing a monitoring method which acts automatically to maintain particularly the water and/or methyl acetate concentrations and which makes it possible to maximize the acetic acid production when the CO feed rate undergoes fluctuations, as will be described below.
It is pointed out first of all that:
The main methanol carbonylation reaction has the following equation:
CO+CH3OHxe2x86x92CH3COOH
The main secondary reactions are:
The water gas reaction, also called WGSR (Water Gas Shift Reaction):
CO+H2Oxe2x86x92CO2+H2
This reaction causes a loss of carbon monoxide with the simultaneous production of hydrogen and carbon dioxide.
It is important to evaluate this secondary reaction quantitatively relative to the main reaction. To use a criterion independent of the production level, one talks of hydrogen or CO2 selectivity, the selectivity being expressed as the ratio of the number of moles of carbon monoxide taking part in the secondary reaction to the total number of moles of carbon monoxide taking part in the main reaction and the secondary reactions.
The propionic acid formation reaction with the following overall equation:
CH3COOH+2H2+COxe2x86x92H2O+CH3CH2COOH
Hydrogen and carbon monoxide are thus consumed.
It may be pointed out here that part of the hydrogen generated by the above water gas reaction is consumed by the reaction to give propionic acid by-product. Thus, when the gas stream at the reactor outlet is analyzed, the hydrogen selectivity and CO2 selectivity are no longer equivalent.
The CO2 selectivity is more representative of the water gas reaction, but, for practical reasons associated with the gas analysis, it is not excluded to use the hydrogen selectivity for controlling the industrial installation.
The acetic acid and methanol esterification reaction to give methyl acetate The methyl acetate formed in this reaction can interfere with the separation of the liquid stream condensed at the top of the first purification column if its concentration increases without control in the reactor.
Depending on the carbon monoxide production process upstream of the acetic acid reactor, the carbon monoxide stream may not be totally constant. Small variations can be damped by installing a carbon monoxide buffer reservoir upstream of the acetic acid reactor, making it possible to reduce the variations in carbon monoxide flow rate and smooth them out over time insofar as the pressure level of the carbon monoxide source is greater than the pressure level at which it is consumed in the carbonylation reactor.
The result of larger fluctuations is that a carbon monoxide stream which can be defined as excess CO cannot be used in the reaction and then has to be discharged into the atmosphere, either via a flare stack or via a combustion system with heat recovery.
The invention consists in varying the reactor temperature and the methanol feed rate in the reactor for the purpose of adjusting the acetic acid production to the quantity of carbon monoxide available, while at the same time maintaining a low hydrogen or CO2 selectivity or conditions which make it possible to optimize the CO consumption, especially by means of a preprogrammed electronic device such as a multivariable controller with predictive control.
Thus, in contrast to the previous modes of operation, where the reactor temperature remained fixed at a given value for long periods, the reactor temperature in the process according to the invention varies so that all the available carbon monoxide can be used.
It became apparent that, even though the parameters used as input variables in the controller were not the same, according to whether the process was one which is operated conventionally with a water content greater than or equal to 14% in the reaction medium, or whether it was a so-called xe2x80x9clow water contentxe2x80x9d process, it was possible in all cases to optimize the acetic acid and/or methyl acetate production by means of a controller acting on the reactor temperature and the methanol feed rate (output variables or action variables) as a function of the carbon monoxide feed rate and at least one additional parameter chosen as a set variable (input values or objective values).
Thus, according to its essential characteristic, the invention relates to a process for monitoring acetic acid and/or methyl acetate production in a continuous preparative process by the carbonylation of methanol or a carbonylatable derivative of methanol with carbon monoxide in the liquid phase, in the presence of water and a homogeneous catalyst system, said preparative process being carried out in an industrial installation comprising:
a zone I, called a reaction zone, comprising a reactor in which said methanol carbonylation reaction is carried out in the liquid phase at a temperature of 150 to 250xc2x0 C., under a pressure of 5xc2x7105 to 200xc2x7105 Pa and with the venting of part of the gaseous canopy above the liquid level of the reaction medium in said reactor;
a zone II, called a vaporization zone or flash zone, in which the liquid originating from the reaction medium in zone I is partially vaporized at a pressure below that of zone I, the liquid fraction originating from this partial vaporization being recycled into the reactor; and
a zone III, called a purification zone, in which the vaporized fraction originating from said flash zone II is distilled on one or more distillation columns, at the outlet of which the acetic acid and/or methyl acetate are recovered, the other constituents of said vaporized fraction being at least partially recycled into said reactor,
wherein the reactor temperature and the feed rate of the methanol or carbonylatable derivative in said reactor are brought under the control of the carbon monoxide feed rate and at least one parameter defining the composition of the reaction medium and/or the vents.
The present invention therefore provides a better means of monitoring acetic acid and/or methyl acetate production in a continuous process for the carbonylation of methanol or a carbonylatable derivative of methanol with carbon monoxide in the liquid phase, in the presence of water and a homogeneous catalyst system, said process being carried out in an industrial installation comprising three main zones I, II and III defined above.
xe2x80x9cCarbonylatable derivative of methanolxe2x80x9d is understood as meaning any of the methanol derivatives conventionally used in industrial processes for the preparation of acetic acid and/or methyl acetate by carbonylation, for example dimethyl ether, methyl halides or methyl acetate.
The monitoring process according to the invention can be carried out by means of any electronic device for assuring the desired servo-control so as to minimize the carbon monoxide losses when the carbon monoxide feed undergoes fluctuations.
In particular, this servo-control device can be an electronic control device preprogrammed for this purpose.
More precisely, the servo-control device acts on the reactor temperature and on the methanol feed rate in the reaction so as to minimize the carbon monoxide losses by continuously monitoring the carbon monoxide feed rate in the reactor and at least one of the parameters defining the composition of the reaction medium and/or the vents.
Those skilled in the art will easily understand that the parameter chosen as the input for the device for effecting the servo-control depends on the mode of operation of the acetic acid and/or methyl acetate manufacturing process, and that, in particular, these parameters may be different according to whether the process is a so-called conventional process of the Monsanto type or a so-called xe2x80x9clow water contentxe2x80x9d process, as will be apparent from the following description.
The control device can be either a monitoring-control system if it possesses the requisite functions, or a multivariable predictive controller, or any other electronic system having the following characteristics:
an input/output (I/O) device;
an interface for digital conversion of the analog inputs/outputs; and
a calculating processor.
However, a multivariable predictive controller was chosen for the examples, on the one hand to escape the specific characteristics associated with monitoring-control systems, and on the other hand to be able to make direct use of the programs supplied with the multivariable controller.
Such a device is based on the use of a mathematical control model and enables a predictive control which, by relying on this mathematical model, produces a hypothesis about the future of the variable to be monitored.
The commercially available multivariable predictive controllers generally incorporate a library of mathematical control models.
Before it can be used, a multivariable predictive controller has to be programmed, which is generally carried out in the following manner:
A mathematical model is chosen from the above-mentioned library as a function of the reactor used and the chemical reaction which it is desired to perform. This choice is typically made empirically by means of preliminary tests which consist in testing all the mathematical models on the chemical reaction in question and observing the control responses obtained.
Values of so-called action variables and corresponding values of so-called objective variables are then supplied to the controller. The action variables are the variables which are to be acted upon so that the objective variables are regulated around desired set values. In the present invention the action variables consist of the reactor temperature and the methanol flow rate and the objective variables consist of the carbon monoxide feed rate and the above-mentioned parameter defining the composition of the reaction medium and/or the vents.
This second phase amounts to the creation, in the controller, of a database representing the relationships between the action variables and the objective variables.
A calculating program, supplied with the controller, then optimizes the control parameters, such as the gain and the lag, which will have to be applied to the controller""s electrical output signals in order to control devices (typically valves) acting on the action variables.
The values of the action variables and objective variables which are supplied to the controller are obtained during a preliminary experimental phase, without control, which consists in carrying out the chemical reaction, rapidly increasing the value of one of the variables by increments so as to cause the system to change, and observing the change in the different variables by measuring their value continuously.
Once the controller has been programmed, it is integrated into the control station and progressively looped into the process.
When the process is carried out, the controller regulates the objective variables around set values by acting on the action variables via the above-mentioned devices.
In a first variant, the invention applies to the monitoring of conventional processes for the carbonylation of methanol to acetic acid and/or methyl acetate with water contents greater than or equal to 14%.
It is well known that, in such a conventional process for the carbonylation of methanol to acetic acid and/or methyl acetate in the liquid phase, catalyzed by rhodium, with water contents greater than or equal to 14%, the carbon monoxide is introduced under the control of the total pressure in the reactor; the methanol is introduced at a fixed rate to give the desired acetic acid production at a fixed reactor temperature.
This monitoring method works if the catalyst is sufficiently active.
If this is not the case, the methanol not converted to acetic acid esterifies to methyl acetate. The increase in methyl acetate concentration in the reactor causes poor performance in the purification zone, particularly at the top of the first purification column, where the increase in methyl acetate concentration first impairs and then inhibits the separation of the condensed liquid into two different liquid phases (a light aqueous phase, part of which serves as column reflux and part of which is recycled into the reactor, and a heavy organic phase, all of which is recycled into the carbonylation reactor).
The production level then has to be reduced in order to correct the situation, after which more catalyst is added or the reactor temperature is increased in order to allow operation at a higher production level.
It has been found that the reaction can be better controlled by monitoring the secondary water gas reaction which produces hydrogen and carbon dioxide.
Continuous or sequential analysis of the plant""s purge gases, coupled with measurement of the total flow rate of the plant""s vents, makes it possible to determine the hydrogen and carbon dioxide flow rates generated by the secondary reactions in the carbonylation reactor.
The ratio of these partial flow rates to the total flow rate of carbon onoxide entering the reactor provides access to the selectivities.
It may be pointed out here that part of the hydrogen generated by the water gas reaction, WGSR, is consumed by the reaction to give propionic acid by-product, so the hydrogen selectivity and CO2 selectivity are no longer equivalent.
The CO2 selectivity is more representative of the water gas reaction, but on certain occasions, for practical reasons associated with the gas analysis, the hydrogen selectivity may be used to control the industrial installation.
Thus, in the first variant of the invention, the reaction is monitored well by varying the flow rate of methanol, or carbonylatable derivative of methanol used, at the reactor inlet, and the reactor temperature, to give a hydrogen or CO2 selectivity less than or equal to 0.01 and/or a methyl acetate concentration in the reactor of less than 5% by weight, preferably of less than 2% by weight, making it possible to assure a good decantation at the top of the first purification column.
The tests performed by the inventor of the present invention have clearly demonstrated that by acting via the controller on the reactor temperature and the CO feed rate, it is possible to limit the CO losses when the carbon monoxide feed rate in the reactor varies, by imposing a set value less than or equal to 0.01 on the CO2 or H2 selectivity and/or by maintaining the methyl acetate concentration at a value of less than 5% by weight, preferably of less than 2%, in the reaction medium.
Thus, in this first variant, where the water concentration in the reaction medium is greater than or equal to 14% by weight, the control involves the CO2 or H2 selectivity and the flow rate of CO to be consumed, and the controller acts both on the reactor temperature and on the flow rate of methanol (or carbonylatable derivative) entering the reactor.
Using the control device in this first variant of the invention made it possible to maintain the CO2 (or hydrogen) selectivity within narrow limits of variation by acting on the reactor temperature and on the feed rate of methanol (or carbonylatable derivative) in the reactor, and made it possible to optimize the carbon monoxide consumption, even in the presence of relatively large fluctuations in the carbon monoxide feed rate in the reactor.
In a second variant, the invention is also applicable to so-called xe2x80x9clow water contentxe2x80x9d processes for the manufacture of acetic acid and/or methyl acetate in the liquid phase, in the presence of a homogeneous catalyst, i.e. to the case where the water concentration in the reaction medium is less than 14% by weight.
It is well known that, in contrast to the conventional processes where the water concentration is greater than or equal to 14% by weight of the reaction medium, the water concentration in such xe2x80x9clow water contentxe2x80x9d processes is a direct parameter of the kinetics of the acetic acid production reaction, whereas the carbonylation reaction is relatively insensitive to the CO2 selectivity; for this reason, those skilled in the art will easily understand that, in such a case, contrary to the previous case, the water concentration in the reaction medium, rather than the CO2 selectivity, will be chosen as a parameter used as an objective variable of the controller.
Therefore, the water concentration, which will be fixed at a predetermined value, and the flow rate of carbon monoxide entering the reactor will advantageously be chosen as objective variables of the controller for the xe2x80x9clow water contentxe2x80x9d manufacturing processes.
It became apparent that, in this case too, it was possible to act via a preprogrammed controller, particularly via a multivariable predictive controller, on the reactor temperature and the flow rate of methanol entering the reactor in order to maintain the water concentration at said predetermined value and to optimize the consumption of carbon monoxide when its feed underwent fluctuations.
In one advantageous variant applicable to both the embodiments of the invention described above, it seemed particularly useful also to bring the flow rate of liquid passing from reaction zone I into flash zone II, and the flow rates of recycle liquid passing from zones II and III into the reactor, under the control of the liquid level in the reactor so that the level remains fixed at a predetermined value.
This predetermined value is advantageously fixed at between 50 and 100% of the absolute total scale of the levels in the reactor.
Thus the controller used according to the invention can also be used to regulate the liquid level in the reactor by adding to the controller, as an action variable, the flow rate of liquid from the reactor into the flash zone and the various return flow rates into the reactor, particularly the liquid flow rates and streams originating from zones II and III.
In another particularly advantageous variant of the invention, the servo-control device used according to the invention, particularly the multivariable predictive controller, can be used to monitor and regulate the water content in the reaction medium.
In particular, it can be used to monitor and regulate on the one hand the equipment for avoiding the accumulation of water in the reaction medium, particularly in the case of a distillation column for extracting water from the process for the preparation of acetic acid.
With the same objective of monitoring the water content of the reaction medium, said device can also be used to monitor and regulate said flow rate or flow rates of methyl acetate, dimethyl ether or acetic anhydride introduced to replace part of the methanol feed, for the purpose of adjusting the water content in the reactor.
In another particularly advantageous variant of the invention, in order to relieve the purification train, it is possible to add exchangers for absorbing part of the heat of the acetic acid production reaction. Monitoring of the reactor temperature is then based on monitoring of the heat exchanged in this way, and it has been possible to show that the system according to the invention does indeed enable all the available carbon monoxide to be used effectively with a good control of the composition in the reactor (methyl acetate).
In another variant of the invention, part of the heat of the acetic acid production reaction can be removed or recovered.
This removal or recovery can be effected either at the reactor outlet via a heat exchanger placed on a loop for recirculating reaction liquid into said reactor, or at the reactor inlet on recycle streams entering said reactor.
In another variant, the fluctuations or variations in the carbon monoxide feed rate can be damped via a buffer reservoir placed upstream of said reactor.
In this variant, a set value of the carbon monoxide flow rate is made to depend on the pressure inside said buffer reservoir.
In another variant of the invention, the fluctuations in the flow rate of carbon monoxide entering the reactor can be damped by discharging at least part of the excess into the atmosphere.
This discharge may be effected particularly in conventional manner, either via a flare stack or via a heat recovery system.
Finally, it became apparent that it was particularly advantageous to couple the control device used according to the invention with an analyzer, operating in the near infrared region, for measuring the concentrations of water and methyl acetate and/or methyl iodide in the reaction medium.
This coupling of a real-time process analyzer based on analysis in the near infrared region was found to be particularly valuable in the xe2x80x9clow water contentxe2x80x9d processes, where it is important to control not only the methyl acetate concentration but also the water content, which has a direct influence on the kinetics of the acetic acid and/or methyl acetate production reaction.
In general, the monitoring process of the invention is applicable to any continuous processes for the manufacture of acetic acid and/or methyl acetate in the liquid phase, in the presence of a homogeneous catalyst system.
It is very particularly applicable to manufacturing processes in which the catalyst system comprises at least one group VIII metal, particularly rhodium, iridium or platinum.
It is also applicable, particularly advantageously, to carbonylation processes in which the catalyst system also comprises at least one co-catalyst, particularly